In the side airbag device shown in following Patent Document 1, it is disposed such that a folded-up portion of a side airbag is positioned at the vehicle transverse direction outer side, and an inflator is positioned at the vehicle transverse direction inner side, so as to interpose the side frame (side frame) of a seatback therebetween. Further, the inflator is disposed at the vehicle rear side of the side frame, and at least a portion of the folded-up portion of the side airbag is disposed at the vehicle front side of the inflator. Due thereto, both an improvement in the design of the seatback and an improvement in the ease of installation and the expansion performance of the side airbag device are achieved.    Patent Document 1: Japanese Patent Application Laid-Open No. 2007-145192